Operation Deepscan
In October 1987 a line of about two dozen boats with echo sounders swept Loch Ness from end to end in the largest sonar search for the monster.
1987 CE · 57.30, -4.45 · status: disputed
The account
The search recorded three sonar contacts at depth that the team could not immediately explain. It also filmed a tree stump that looked very like an earlier 'gargoyle head' underwater photo.
The sceptical view
Sonar experts suggested the contacts were seals, debris, thermal layers or boat wakes; no animal was found.
The researched reading
The largest sonar sweep of Loch Ness found a few unexplained echoes and no monster. Fish, seals, debris and water layers explain the contacts far better than an unknown animal.
| Explanation | Weight | Why |
|---|---|---|
| Something known, misidentified | 50% | Three mid-water sonar contacts from a large sweep of a deep, cold loch fit fish shoals, seals that occasionally enter the loch, debris, boat wakes or thermocline echoes; sonar experts proposed these at the time. |
| A cultural story | 20% | Operation Deepscan was a media event built around the legend; ambiguous echoes became "unexplained contacts" because of the monster story. |
| Genuinely anomalous | 14% | The three contacts were never identified, which leaves some room; but unidentified sonar echoes in a large loch are common and not evidence of something unknown to science. |
| Perception, memory or mind | 10% | Searchers expecting a monster interpret uncertain echoes generously, a known bias in sonar searches. |
| An unknown or out-of-place animal | 4% | A population of large animals would be needed and would have to feed on the loch’s limited fish stock; a 2018 environmental-DNA survey found no reptile DNA but abundant eel DNA, leaving at most a large eel. |
| A hoax or fabrication | 2% | The sweep was conducted openly with many crews; there is no sign of faking. |
Official position: No official investigation; the organisers reported three unexplained contacts and found no animal.